1. Technical Field
The present invention relates to polymer electrolyte fuel cells, and relates to separators for forming gas passages in fuel cells.
2. Background Art
In polymer electrolyte fuel cells, a separator is layered on both sides of a plate-shaped membrane electrode assembly to form a unit of a layered structure, and plural units are layered to form a fuel cell stack. The membrane electrode assembly is a three-layered structure, in which a polymerized electrolyte membrane made from a resin such as ion-exchange resin is held by a pair of gas diffusion electrode plates (positive electrode plate and negative electrode plate). The separator is formed with gas passages for flowing a gas between the gas diffusion electrode plate and the separator. According to the fuel cell, hydrogen gas as a fuel is provided to the gas passages facing the gas diffusion electrode plate at the negative electrode side, and an oxidizing gas such as oxygen or air is provided to the gas passages facing the gas diffusion electrode plate at the positive electrode side, whereby electricity is generated by electrochemical reaction.
The separator provides electrons generated by catalytic reaction of the gas at the negative electrode side (hydrogen gas) to an external circuit, and transfers electrons provided from the external circuit to the positive electrode side. Therefore, separators are made from conductive materials of the carbon type and the metal type. In particular, it has been mentioned that material of the metal type is advantageous since it has good mechanical strength. Separators of the metal type are generally made by press forming a thin plate of aluminum alloy, carbon steel, or stainless steel to a corrugated sheet, of which the design has been disclosed in Japanese First Publications Nos. 2000-223137 and 2000-138065.
Fuel cells are assembled into a fuel cell stack by layered membrane electrode assemblys and separators as mentioned above. In the layering condition, plural protrusions projecting toward one surface side and another surface side of the separator are press contacted with an electrode of the membrane electrode assembly. The surface pressures of the protrusions press contacted with the electrode are required to be uniform as possible to reduce the internal resistance of the cell as effectively as possible so as to improve the power generation characteristics. However, since the metallic separator is a press formed product in a thin plate, it has been difficult to sufficiently satisfy such a requirement. Therefore, the surface pressure with respect to the electrode may be large in some protrusions, but may be small in other protrusions, and additionally, the surface pressure may vary according to the position in a protrusion. Thus, the surface pressure at the protrusions is often variable. The variation in the surface pressure results in degradation of power generation characteristics, and is a problem to be solved.